RE: Partial aggregates pushdown

Hi Mr.Bruce, Mr.Pyhalov, hackers.

Thank you for comments. I will try to respond to both of your comments as follows.
I plan to start revising the patch next week. If you have any comments on the following
respondences, I would appreciate it if you could give them to me this week.

> From: Bruce Momjian <bruce(at)momjian(dot)us>
> Sent: Saturday, June 10, 2023 1:44 AM
> I agree that this feature is designed for built-in sharding, but it is possible people could be using aggregates on partitions
> backed by foreign tables without sharding. Adding a requirement for non-sharding setups to need PG 17+ servers might
> be unreasonable.
Indeed, it is possible to use partial aggregate pushdown feature for purposes other than sharding.
The description of the section "F.38.6. Built-in sharding in PostgreSQL" assumes the use of
Built-in sharding and will be modified to eliminate this assumption.
The title of this section should be changed to something like "Aggregate on partitioned table".

> From: Bruce Momjian <bruce(at)momjian(dot)us>
> Sent: Saturday, June 10, 2023 1:44 AM
> Looking at previous release note incompatibilities, we don't normally change non-administrative functions in a way that
> causes errors if run on older servers. Based on Alexander's observations, I wonder if we need to re-add the postgres_fdw
> option to control partial aggregate pushdown, and default it to enabled.
>
> If we ever add more function breakage we might need more postgres_fdw options. Fortunately, such changes are rare.

I understand what the problem is. I will put a mechanism maintaining compatibility into the patch.
I believe there are three approaches.
Approach 1-1 is preferable because it does not require additional options for postgres_fdw.
I will revise the patch according to Approach 1-1, unless otherwise commented.

Approach1:
I ensure that postgres_fdw retrieves the version of each remote server
and does not partial aggregate pushd down if the server version is less than 17.
There are two approaches to obtaining remote server versions.
Approach1-1: postgres_fdw connects a remote server and use PQserverVersion().
Approach1-2: Adding a postgres_fdw option about a remote server version (like "server_version").

Approach2:
Adding a postgres_fdw option for partial aggregate pushdown is enable or not
(like enable_partial_aggregate_pushdown).

Sincerely yours,
Yuuki Fujii

--
Yuuki Fujii
Information Technology R&D Center Mitsubishi Electric Corporation